Key Color still shows in sort of a torn manner
The main problem is that the alpha channel generated by the keying node has to be associated with the picture. Use an Alpha Convert node in straight to premultiply mode.

The artifacts at the top of the image are caused by the compression on the original image. Some compression algorithms for video work by considering similar colors to be one solid block of color, usually this is not as visible, but since you are using this area for keying, the macro-blocks become visible.

What that means is that the color you are keying out is not even.
The solution are:

Use an image that has less or no compression.

Change the threshold of tolerance for the key, either by modifying the Key Color or by modifying the values for Clip Black (pixels that are considered part of the background) or Clip White (pixels that are considered foreground)
Read the manual for this node:


https://docs.blender.org/manual/en/latest/compositing/types/matte/keying.html